Yojijukugo (Japanese: 四字熟語) is a Japaneselexeme consisting of four kanji (Chinese characters). English translations of yojijukugo include "four-character compound", "four-character idiom", "four-character idiomatic phrase", and "four-character idiomatic compound". It is equivalent to the Chinese chengyu.

The definition of yojijukugo is somewhat murky since the Japanese word jukugo (熟語, literally "ripe/mature/popular word") can linguistically mean "compound", "idiom", or "phrase".

Yojijukugo in the broad sense simply means any Japanese compound words consisting of four kanji characters. In the narrow or strict sense, however, the term refers only to four-kanji compounds that have a particular (idiomatic) meaning that cannot be inferred from the meanings of the components that make them up.

There exists a very large number — perhaps tens of thousands — of four-character compounds. A great majority of them are those whose meanings can be easily deduced from the literal definition of their parts. These compounds may be called non-idiomatic yojijukugo.

For example, the compound word 屋内禁煙 okunaikin'en "no smoking indoors" is a non-idiomatic yojijukugo. It is made up of four characters: 屋 oku building, 内 nai inside, 禁 kin prohibited, and 煙 en smoking. Alternatively, it can be regarded as consisting of two common two-character compounds: 屋内 okunai indoors, and 禁煙 kin'en prohibition of smoking. Either way, the meaning of the compound is clear; there are no idiomatic meanings beyond the literal meanings of its components. Below are a few more examples of non-idiomatic yojijukugo:

By contrast, several thousands of these four-character compounds are true idioms in the sense that they have a particular meaning that may not be deducted from the literal meanings of the component words. An example of the highly idiomatic compound is:

"Ocean-thousand, mountain-thousand" means "a sly old fox" or someone who has had all sorts of experience in life so that s/he can handle, or wiggle out of, any difficult situations through cunning alone. This meaning derives from an old saying that a snake lives in the ocean for a thousand years and in the mountains for another thousand years before it turns into a dragon. Hence a sly, worldly-wise person is referred to as one who has spent "a thousand years in the ocean and another thousand in the mountain".

Many idiomatic yojijukugo were adopted from classical Chinese literature. Other four-character idioms are derived from Buddhist literature and scriptures, old Japanese customs and proverbs, and historical and contemporary Japanese life and social experience. The entries in the published dictionaries of yojijukugo are typically limited to these idiomatic compounds of various origins.

The Japanese yojijukugo are closely related to the Chinese chengyu in that a great many of the former are adopted from the latter and have the same or similar meaning as in Chinese. Many other yojijukugo, however, are Japanese in origin. Some examples of these indigenous Japanese four-character idioms are: